Conventionally, so-called Ziegler-Natta catalysts which consist of a titanium or vanadium compound as primary catalyst component and an alkylaluminium compound as cocatalyst component have been usually employed for preparing ethylene homopolymers or copolymers of ethylene with α-olefins. Though a Ziegler-Natta catalyst system exhibits high activity on ethylene polymerization, the catalyst system is disadvantageous in that the molecular weight distribution of the produced polymer is broad owing to irregular catalyst activation point, and it may result in irregular distribution of composition, particularly in copolymers of ethylene with α-olefin.
Recently, metallocene catalyst systems consisting of a metallocene compound of Group 4 transition metal in the Periodic Table of Elements, such as titanium, zirconium and hafnium, and methyl aluminoxane as a cocatalyst have been developed. Since the metallocene catalyst system is a homogeneous catalyst having a monomodal catalyst activation point, it can provide polyethylene having narrow molecular weight distribution and homogenous composition distribution as compared to conventional Ziegler-Natta catalyst. For example, European Patent Publication Nos. 320,762 and 3,726,325; Japanese Patent Laid-Open Nos. Sho 63-092621, Hei 02-84405 and Hei 03-2347 reported that ethylene can be polymerized with high activity by activating the metallocene compounds such as Cp2TiCl2, Cp2ZrCl2, Cp2ZrMeCl, Cp2ZrMe2, ethylene(IndH4)2ZrCl2 by using methyl aluminoxane as cocatalyst, to provide polyethylene having the molecular weight distribution (Mw/Mn) in the range from 1.5 to 2.0. However, it is difficult to obtain polymers of high molecular weight by using such a catalyst system. Particularly, when the catalyst system is applied to solution polymerization carried out at a high temperature of 140° C. or higher, the polymerizing activity abruptly decreases but β-dehydrogenation predominates, so that the system is known to be not suitable for preparing polymers having high molecular weight (weight average molecular weight, Mw) of 100,000 or more.
In the meanwhile, disclosed were so-called geo-restrictive non-metallocene type catalysts (also referred to as single activation point catalysts), wherein the transition metals are linked in the form of a ring, as catalysts for preparing high molecular weight polymers with high catalytic activity in polymerization of ethylene homopolymers or copolymerization of ethylene with α-olefin. European Patent No. 0416815 and 0420436 suggested the examples wherein amide groups are linked in the form of a ring to one cyclopentadiene ligand, while European Patent No. 0842939 showed exemplary catalysts wherein phenolic ligands (as electron donors) are linked to cyclopentadiene ligand in the form of a ring.
However, there are many difficulties to commercially utilize such catalysts since the yield of the procedure of ring formation between the ligands and the transition metal compounds is very low during the synthesis of the geo-restrictive catalyst as described above.
On the other hand, examples of non-metallocene catalysts that are not geo-restrictive can be found in U.S. Pat. No. 6,329,478 and Korean Patent Laid-Open No. 2001-0074722. It is found that the catalyst of single activation point, which employs a phosphinimine compound as a ligand, showed high ethylene conversion in the copolymerization of ethylene with α-olefin under the condition of solution polymerization at a high temperature of 140° C. or more. U.S. Pat. No. 5,079,205 discloses the examples of catalysts containing bis-phenoxide ligand, and U.S. Pat. No. 5,043,408 those containing bisphenoxide ligand of chelate type. However those catalysts have so little activity that can be hardly employed for industrial preparation of ethylene homopolymers or ethylene copolymers with α-olefin, which is carried out at a high temperature.